In a conventional wireless network of nodes that use a protocol stack, internet protocol (IP) messages 800, see FIG. 8, are transferred between an application layer, a transport control protocol (TCP) layer, a network layer, a link layer, a media access (MAC) layer and a physical (PHY) layer. Outgoing messages are transferred top to bottom, and incoming message are transferred bottom to top.
The TCP layer is responsible for end-to-end congestion control. The TCP layer relies on message acknowledgement for rate management.
The MAC layer uses carrier sense multiple access (CSMA) and random back-off to control access to the physical layer as specified by the IEEE 802.11 standard. If two or more nodes transmit messages concurrently, collisions can occur so that the messages cannot be decoded. The performance of the network, e.g., throughput, error rate, and delay, depend on the collision rate.
The nodes can be embedded in vehicles to disseminate information related to vehicular environments. The IEEE P1609 standard, which pertains to wireless access in a vehicular environment (WAVE), specifies a WAVE protocol stack.
In vehicular networks, multiple priority levels are defined. Time-sensitive short messages (SM) 900, see FIG. 9, should be reliably sent with low latency to all nearby vehicles and road-side infrastructure. As an example, the Society of Automotive Engineers (SAE) has defined a set of safety critical short messages such as crash-pending notification, and hard braking, only tolerates a latency of up to 10 milliseconds. This criterion can not be satisfied in a congested network where a large number of messages need to be communicated within a relatively short time interval.
In both unicast and broadcast networks, an Enhanced Distributed Coordination Function (EDCF) supports different access priorities to different access categories (ACs) at the MAC layer. High priority AC achieves shorter delivery latency, and higher throughput.
Message collision rate, message delivery delay, and medium utilization are described by Romdhani et al., “Adaptive EDCF: enhanced service differentiation for IEEE 802.11 wireless ad-hoc networks,” Wireless Communications and Networking, 20 Mar. 2003. The collision rate and delay increases as a function of the overall load of in wireless networks.
A message rate decreasing procedure is described by Yang et al., “vehicle-to-vehicle communication protocol for cooperative collision warning,” Mobile and Ubiquitous Networks: Networking and Services, 2004. That procedure controls the message rate at which alert messages are generated by each vehicle to achieve low latency in dense vehicular networks.